1. Field of the Invention
The invention relates to a process for spinning high molecular weight poly (alpha-olefin) (UHMW PO) filament from solution in a volatile solvent with recovery and recycling of the solvent. More particularly, the invention relates to a process for spinning high molecular weight polyethylene homopolymer or copolymer filament from solution in a volatile solvent with recovery and recycling of the solvent.
2. Description of the Related Art
High strength, high modulus polyethylene and polypropylene filaments have been prepared from ultrahigh molecular weight polymers by solution spinning and drawing. Such filaments have proven extremely useful in ballistic protection, penetration- and cut-resistant applications, composites, ropes and netting among other applications. The first description of solution spinning of strong polyethylene filaments, was by P. Smith, P. J. Lemstra, B. Kalb and A. J. Pennings, Poly. Bull., 1, 731 (1979). Single filaments were spun from 2 wt. % solution in decalin (decahydronaphthalene) at 130° C., quenched to a gel state, and then stretched while evaporating the decalin in a hot air oven at 100 to 140° C.
In the intervening decades since this first description, two principal solution spinning processes have been developed differing in part by whether the spinning solvent is volatile or non-volatile at the spinning temperature. In one solution spinning process, a solution of ultrahigh molecular weight polyethylene (UHMW PE) is prepared at elevated temperature in a first solvent such as a heavy mineral oil that is essentially non-volatile at the spinning temperature. The polymer solution is spun and quenched to a gel state essentially without evaporation of the spinning solvent. The non-volatile spinning solvent is removed from the gel fibers by extraction with a second solvent having a normal boiling point less than 100° C. such as trichlorotrifluoroethane. The gel filaments containing the second solvent are dried to form xerogel filaments. Stretching of the filaments occurs in the gel state containing the first solvent, in the gel state containing the second solvent and in the dry xerogel filament state. Variations of this basic process are described for example in U.S. Pat. Nos. 4,413,110; 4,455,273; 4,536,536; 4,551,296; 4,584,347; 4,663,101; 5,736,244; 5,972,498; and 6,448,359B1.
In this process both the non-volatile spinning solvent and the low boiling second solvent are recovered and recycled, e.g., through sequence tub washing or counter-current flow. Exiting the extraction device are a mixed solvent stream and the gel filament. The gel filament exiting the extraction device is composed of the UHMW PE swollen with the second solvent.
The gel filament containing the low boiling second solvent is passed into one or more drying chambers where the second solvent is evaporated into an air or inert gas stream. Upon leaving the drying chambers, the gas stream containing the second solvent is passed into a solvent recovery unit, e.g., a chemical adsorption, or condensation unit or for non-recovery processes, catalytic oxidation or burning.
The other stream leaving the extraction device consisting of the mixed solvents, i.e., the non-volatile spinning solvent and the low boiling extraction solvent, is passed into a one-stage still where a major portion of the low boiling second solvent is evaporated. The overhead product from the still, consisting of essentially pure second solvent is condensed and recycled to the extraction device. The bottoms product is separated by conventional means into the low boiling second solvent and the non-volatile spinning solvent. The low boiling solvent is then recycled to the extraction device. The non-volatile spinning solvent is recycled to the device in which the UHMW PE solution is formed.
U.S. Pat. No. 5,741,451 describes a method for drawing UHMW PE filaments such as have been prepared by this process. U.S Pat. No. 2004/0040176 A1 describes an oven and process suitable for simultaneously drawing and drying “lowly oriented polyethylene” filaments containing a solvent, apparently describing the gel filaments containing the extraction solvent. Paragraph [0013] of that application refers to a “gas purification installation” without further description.
The second principal UHMW PE solution spinning process closely follows the disclosure of Smith, Lemstra, Kalb and Pennings cited above. In this process, a solution of UHMW PE is prepared in a solvent, typically decalin, that is volatile at the spinning temperature. The UHMW PE solution is spun at elevated temperature, cooled to form gel filaments, and the spinning solvent is evaporated during cooling and while the filaments are stretched. Variations on this basic process are described, for example, in GB 2,042,414, U.S. Pat. Nos. 4,411,854; 4,422,993; 4,430,383; 4,436,689; 5,443,904; 5,547,626; 6,605,348 B1 and JP-A-60/5264. However, none of the patents or publications related to this process discusses recovery or recycling of the spinning solvent.
An article titled, “Organische Dämpfe in Luft und Stickstoff—Enstsorrgen oder Wertstoffe zurückgewinnen?” (translated as “Organic Vapors in Air and Nitrogen—Disposal or Recovery of Potentially Valuable Recyclables?”), by H. Forster, Erdöl Erdgas Kohle, 114(No. 4), 211-214, 1995, discusses methods for disposal or recovery of solvent vapors in general, without specific reference to filament spinning processes. The methods of choice are differentiated by the concentration of solvent in the gas stream. Disposal by combustion is indicated for solvent contents less than 60 g/m3. Recovery by condensation or membrane concentration followed by condensation is indicated for solvent contents in the range of 100-1500 g/m3. Adsorption is indicated at both low (<20 g/m3) and high (>30 g/m3) solvent contents.
It will be recognized that a process of spinning UHMW PO from a volatile spinning solvent avoids the need for an extraction solvent and the several separation steps that its use entails. However, a need exists for a solution spinning process for UHMW PO using a volatile spinning solvent that recovers and recycles the spinning solvent by simple, economical means.